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The present invention relates to syndiotactic polypropylene blends having good optical clarity and an increased level of impact-resistance or improved mechanical toughness. More particularly, the present invention relates to a syndiotactic polypropylene (sPP) which has been blended with an ultra low density polyethylene (ULDPE) and, optionally, a random copolymer of isotactic polypropylene (iPP) and ethylene. Additionally, the syndiotactic polypropylene blend of the present invention may be subsequently formed into films, sheets, molded articles or the like.
Polypropylene materials, formed by Ziegler-Natta or metallocene catalysts, are among the most versatile and commonly used thermoplastics in the world today. Polypropylene materials are useful in creating a great variety of finished goods including cast and blown films, injection molded parts, blow molded articles, thermoformed sheets, and fibers which may be subsequently spun or woven to create carpet and other finished goods. Although both polyethylene and polypropylenes are types of polyolefins, polypropylenes tend to be stiffer and exhibit higher yield stresses and melting points in comparison with polyethylenes but are also more prone to fracture, especially at low temperatures. This primarily results from higher glass transition temperatures and may be addressed by producing a toughened blend using rubber or other polymeric impact modifiers to improve low temperature impact resistance at some sacrifice in modulus.
As noted earlier, nearly all commercial grade polypropylenes are produced using either Ziegler-Natta or metallocene catalysts mechanisms. These catalysts allow a certain degree of control in regard to the polypropylene""s tacticity or arrangement of methyl groups extending from the carbon chain backbone of the finished polymer. A polypropylene molecule having a random arrangement of these pendant groups would be known as atactic. Whereas a polypropylene chain which always located the pendant group on the same side of the chain or in the same orientation would be known as isotactic, and one in which the pendant group alternated from one side of the chain to the other in a repeating pattern would be referred to as syndiotactic.
Traditionally, commercial polypropylenes have been isotactic as these tend to exhibit greater strength and stiffness in the finished product. However, relatively recent innovations in catalyst chemistry have enabled relatively large scale operations for the production of syndiotactic polypropylene. Although not as strong or as stiff as isotactic polypropylenes, syndiotactic polypropylenes offer a unique set of properties including greater flexibility, higher resistance to impact, and superior optical clarity.
There are a number of unique applications which are ideally suited to strong, flexible and substantially clear polyolefins. By way of example only, plasticized polyvinyl chloride (PVC) has traditionally been used either alone or with other polymer components to form a number of medical articles including bandages, surgical dressings, and intravenous (IV) solution bags. Plasticized PVC films possess many desirable properties including easy stretch, high degree of recovery, low fatigue and minimal permanent set. However, plasticized PVC film has become less desirable because of know or suspected carcinogens associated with both the PVC monomer and the various plasticizers used in its production. Clearly, in medical articles, food storage containers, and other applications where polymers are either in direct contact with blood or other bodily fluids or in contact with food or other items which are to be ingested or taken into the body, it would be desirable to replace materials like plasticized PVC film with various polyolefins, particularly those with very low extractable contents.
Although syndiotactic polypropylene offers superior strength and optical clarity in comparison with less expensive polyolefins, namely polyethylene, sPP homopolymer is typically lacking in mechanical toughness and impact strength at relatively low temperatures. Additionally, while it is possible to produce polypropylene copolymers which exhibit improved cold temperature impact strengths, these materials do not offer the same amount of optical clarity as sPP homopolymer. Accordingly, there is need for syndiotactic polypropylene blends which offer improved mechanical toughness and cold temperature impact resistance while offering optical clarity levels which are comparable to or better than many sPP homopolymers.
Syndiotactic polypropylene blends prepared in accordance with the present invention address the needs set forth hereinabove and present a rather unique set of mechanical properties by blending syndiotactic polypropylene with an ultra low density polyethylene and, optionally, an isotactic polypropylene random copolymer. In short, the syndiotactic polypropylene blends of the present invention have been found to have improved impact strength, reduced haze, increased light transmittance, and reduced flexural modulus.
More specifically, it has been determined that the addition of about 10 to about 50 wt % of an ultra low density polyethylene plastomer to a syndiotactic polypropylene can greatly enhance mechanical toughness, particularly at low temperatures, while maintaining good optical clarity. These results are made possible by using a sPP material with good optical clarity and dispersing the ULDPE plastomer uniformly throughout the sPP matrix to act as a sort of impact modifier. Moreover, the addition of about 10 to about 50 wt % of an iPP random copolymer to the sPP matrix phase provides comparable or improved mechanical properties while also resulting in a significant reduction in injection molding cycle times.
It is further within the scope of the present invention to provide a method of making syndiotactic polypropylene blends offering improved impact strength and good optical clarity. This may be done by providing a syndiotactic polypropylene material in the form of pellets or fluff and mechanically compounding or blending this material with an ultra low density polyethylene, an iPP random copolymer, or both. This compounding step may be carried out by tumble blending the sPP fluff with the ULDPE pellets and/or the iPP random copolymer, and subsequently feeding this mixture into an extruder or the like to mechanically shear the components into a fairly uniform molten polymer blend. Following extrusion, the syndiotactic polypropylene blends according to the present invention may be further processed according to accepted practices to make cast films, blown films, co-extruded films, laminated sheets, injection molded parts, blow molded containers, and other articles using basic plastic fabrication techniques as known in the art.
Not applicable.